Localization of two enzymes of the tetrahydrobiopterin biosynthetic pathway in embryonic chick retina.

Tetrahydrobiopterin (BH4) is an essential co-factor for the biosynthesis of catecholamine-type neurotransmitters and of nitric oxide (NO). The expression of the enzymes catalyzing the first two steps of the BH4 biosynthetic pathway was studied in the developing chicken retina by in situ hybridization and immunocytochemistry. GTP-cyclohydrolase-I (GTP-CH-I) and 6-pyruvoyl-tetrahydropterin synthase (PTPS) were already expressed in the undifferentiated and proliferating retina of E7. At stage E11 both enzymes were expressed in photoreceptors, amacrine cells, displaced amacrine cells, and ganglion cells, and in the plexiform layers in which synaptic connections take place. At stage E18 the labeling was comparable to E11 but appeared to be more concentrated in photoreceptors and ganglion cells.

Anti-semaphorin 3A antibodies rescue retinal ganglion cells from cell death following optic nerve axotomy.

Damage to the optic nerve in mammals induces retrograde degeneration and apoptosis of the retinal ganglion cell (RGC) bodies. The mechanisms that mediate the response of the neuronal cells to the axonal injury are still unknown. We have previously shown that semaphorins, axon guidance molecules with repulsive cues, are capable of mediating apoptosis in cultured neuronal cells (Shirvan, A., Ziv, I., Fleminger, G., Shina, R., He, Z., Brudo, I., Melamed, E., and Brazilai, A. (1999) J. Neurochem. 73, 961-971). In this study, we examined the involvement of semaphorins in an in vivo experimental animal model of complete axotomy of the rat optic nerve. We demonstrate that a marked induction of type III semaphorin proteins takes place in ipsilateral retinas at early stages following axotomy, well before any morphological signs of RGC apoptosis can be detected. Time course analysis revealed that a peak of expression occurred after 2-3 days and then declined. A small conserved peptide derived from semaphorin 3A that was previously shown to induce neuronal death in culture was capable of inducing RGC loss upon its intravitreous injection into the rat eye. Moreover, we demonstrate a marked inhibition of RGC loss when axotomized eyes were co-treated by intravitreous injection of function-blocking antibodies against the semaphorin 3A-derived peptide. Marked neuronal protection from degeneration was also observed when the antibodies were applied 24 h post-injury. We therefore suggest that semaphorins are key proteins that modulate the cell fate of axotomized RGC. Neutralization of the semaphorin repulsive function may serve as a promising new approach for treatment of traumatic injury in the adult mammalian central nervous system or of ophthalmologic diseases such as glaucoma and ischemic optic neuropathy that induce apoptotic RGC death.